1. Field of the Invention
The invention relates generally to a method of reclaiming a substrate wafer from a semiconductor wafer. The reclaimed substrate wafer is of sufficiently high quality to meet many standards for "prime" wafer substrates used by the semiconductor circuit manufacturers.
2. Discussion of the Background
Semiconductor circuit manufacturers require two qualities of crystalline silicon wafers to satisfy their production requirements: "prime" quality wafers for use in constructing actual semiconductor products; and "test" quality wafers for use to prequalify manufacturing processes for their satisfactory performance, "Prime" wafers are sold to higher quality standards than "test" wafers. "Test" wafers that exhibit quality standards close to that of "prime" wafers are preferred by semiconductor companies and are sold at a higher price than standard quality "test" wafers,
A typical used semiconductor wafer will comprise a silicon substrate wafer with semiconducting components implanted and/or diffused into one surface thereof (hereinafter called active surface) and layers of conducting and insulating materials formed on the surfaces of the silicon substrate wafer. There are several methods in the prior art for reclaiming substrate wafers from used semiconductor wafers.
In U.S. Pat. No. 3,559,281 issued to Mayberry et al, there is described a method of first removing the external conducting and insulating layers to expose the substrate wafer having semiconductor components diffused in an active surface thereof, followed by forming a passivation layer on the active surface of the substrate wafer. With this method the passive surface, i.e. that surface opposite the surface in which the semiconductor components are diffused, was adopted as the future working surface. This passivation layer was designed to keep the diffused impurities in the active surface of the silicon substrate wafer away from the future working surface. This method was not commercially successful because semiconductor manufacturers were fearful of the passivation layer becoming flawed with a resultant release of impurities to the working surface.
The method described in U.S. Pat. No. 3,923,567 employs a combination of gettering and etching steps to remove the impurities. In brief, it involved the diffusion of phosphorus into the surfaces of the stripped silicon substrate wafer at high temperatures. In this way getter sites were created in the regions near the surfaces and impurities were attracted towards these getter sites. It then employs an etching step to remove those undesired portions from the surface of the silicon substrate wafer. In this method the active surface was adopted as the future working surface and was polished. The surface opposite the polished surface i.e., the passive surface was then ground to introduce controlled amounts of crystalline lattice strain. This method produced a reclaimed silicon substrate of relatively high purity compared to the prior methods. However, due to the differential etching rate among different semiconductor components the etching process tended to create a wavy surface which meant that it was required to remove a considerably large amount of stock silicon in the subsequent polishing step, resulting in a silicon substrate wafer of significantly reduced thickness.
U.S. Pat. No. 3,905,162 describes a method of grinding for inducing controlled amounts of strain. However this grinding method was designed to be used as a finishing process in the production of substrate wafers prior to device fabrication and was not used as a method for removing the semiconducting components located within the substrate wafer. Furthermore the particular method of grinding described leaves a substrate wafer having a surface characterized by a "circular" surface roughness pattern. The reclaimed substrate wafer thus ground had the disadvantage that it tended to warp when subsequently subjected to a heat treatment by the semiconductor manufacturer.
The prior art also describes a method of wafer backside grinding used to remove hundreds of microns of silicon to transform a finished semiconductor product wafer of starting thickness near 650 microns to a final wafer thickness near 250 microns so that the wafer's semiconductor products will have a thickness compatible with the depth of the cavity of the product's final encapsulating package. Thus, the purpose of grinding in the back grinding application is different from those of the wafer reclamation procedure in that the back grinding procedure must remove vast amounts of material rapidly while resulting in a final wafer thickness without introducing significant crystal strain. The objectives of the back grinding procedure are met through using a combination of "coarse" abrasive grinding with a large particle size near 120 microns to remove silicon rapidly, followed by a "fine" abrasive grinding with a small particle size near five microns to remove much of the strained silicon. The objectives of the back grinding procedure cannot be met through the use of a single fixed abrasive wheel because the silicon removal rates would be too slow with a small particle abrasive, and the amount of crystal strain would be too great with a large particle abrasive.
Semiconductor manufacturers have now identified an industry need for high quality substrate wafers reclaimed from used semiconductor wafers. More specifically, it is desired that the reclaimed substrate wafers meet the strict flatness specifications for "prime" wafers, have designed in getter sites to keep impurities away from the working surface and have a thickness only slightly less than that of the original substrate wafer. None of the substrate wafers reclaimed by the above methods meet all these requirements.